Single Opt-in Contact Form

We have a website that is members-only. On the page we've created for visitors who are not logged in, we have a MailChimp form that allows them to request an invitation to this site. However, when a visitor fills out the form, MailChimp then requires him to click a link an email to confirm that he requested to be put on the list (double opt-in). We need to remove this double opt-in, and create a page so that when a user submits his request for an invite, he's taken to a Thank you page, which just says thanks and has a link back to the previous page.

We would prefer to continue using mailchimp but if there is another way to do this and store request data in a database, that's fine too.